1. Field of the Invention
The present invention relates generally to the field of downhole sampling and in particular to derivative spectrometry in a downhole environment.
2. Summary of the Related Art
Oil companies take samples from potential hydrocarbon-bearing formations to determine a formation""s propensity to produce hydrocarbons. Oil companies desire the most accurate measure of sample contamination percentage in real time as they are pumping fluid from a formation so that they can decide when to divert a sample being pumped to a sample collection tank. As formation fluid is pumped from the formation, the percentage of filtrate contained in the formation fluid sample diminishes in the pumped fluid. Thus, an oil company typically pumps until a pure sample, relatively free of filtrate can be obtained in order to accurately appraise the hydrocarbon producing potential of the surrounding formation. The oil company does not, however, want to pump unnecessarily long and waste very expensive rig time. Conversely, they do not want to pump too little and collect a useless sample, which is full of contaminants and does not reflect the properties of the formation fluid. If the contamination of filtrate contained in the sample is more than about 10%, the sample may be useless for its intended purpose. Moreover, it may not be discovered that the sample is useless until the sample is retrieved at the surface, making a return trip downhole necessary to collect another sample. In such cases, the PVT properties indicative of formation and formation fluid properties that are measured in the lab cannot be corrected back to true reservoir conditions because of excessive contamination. It is therefore desirable to perform sample contamination measurements downhole. One method of investigation is to use a spectrometer to perform optical measurements on the fluid samples collected in a downhole environment.
Numerous factors can affect downhole spectrometer measurements. In the downhole environment, photodetectors operate at high ambient temperatures and thus are very noisy and produce a substantially diminished signal. Also, contaminated samples consisting of flowing streams of crude oil containing scatterers such as sand particles or bubbles tend to add noise to the system. These scatterers cause the optical spectrum to momentarily xe2x80x9cjumpxe2x80x9d up (get darker) as they pass through the sample cell. At high concentrations, these scatterers cause the measured spectrum to move or jump constantly. To first order, the effect of the scatters is just a momentary baseline offset. An operator can greatly improve the signal-to-noise ratio of a downhole spectrometer by modulating the wavelength of light and using a lock-in amplifier. Thus, there is a need for a spectrometer that operates in a downhole environment and diminishes the effects of the scatterers and the associated offsets.
Spectrometers typically disperse white light into constituent colors. The resulting rainbow of colors can be projected through a sample to be analyzed and onto a fixed array of photodetectors which sense light projected though the sample. Alternatively, by rotating a dispersive element (i.e. grating, prism), the rainbow can be mechanically scanned past a single photodetector one color at a time. In either case, an operator can obtain a sample""s darkness versus wavelength, in other words, the sample""s spectrum.
Photodetectors and their amplifiers always have some thermal noise and drift, which limit the accuracy of a spectral reading. As temperature increases, noise and drift increase dramatically higher at the same time that photodetector signal becomes significantly weaker. If an operator oscillates the wavelength (color) of light about some center wavelength, then the operator can reject most photodetector and amplifier noise and drift by using an electronic bandpass filter that passes only that electrical frequency at which the wavelength of light is being oscillated. The operator can further reject noise by using a phase-sensitive (xe2x80x9clock-inxe2x80x9d) amplifier that not only rejects signals that have the wrong frequency but also rejects signals that have the correct frequency but do not have a fixed phase relationship (indicative of noise) relative to the wavelength oscillation. A lock-in amplifier can improve signal to noise by as much as 100 db, which is a factor of 10100 db/10 or 10 billion.
The output of the lock-in amplifier used in this procedure is proportional to the root-mean-square (RMS) amplitude of that portion of the total signal, which is at the same frequency and has a fixed phase relationship relative to the optical frequency being observed. The more that the darkness of the sample changes with color, the larger this RMS value will be. Thus, the output of lock-in amplifier for a system with an oscillating-wavelength input is proportional to the derivative of the spectrum (with respect to wavelength) at the center wavelength of the oscillation.
A spectrometer based on an oscillating-wavelength and a lock-in amplifier can be used to obtain high accuracy spectral measurements as described in the related art below. U.S. Pat. No. 4,070,111 entitled Rapid Scan Spectrophotometer, by Harrick, Jan. 24, 1978 discloses a spectrophotometer capable of rapid spectral scanning by mounting a low inertia reflective grating directly on the output shaft of a galvanometer-type optical scanner, and sweeping the beam dispersed from the grating across a spherical mirror and after reflection there from across a beam exit slit. The invention also describes rapid wavelength switching for a laser spectrometer.
U.S. Pat. No. 4,225,233 Rapid Scan Spectrophotometer, by Ogan Sep. 30, 1980 discloses a spectrometer capable of providing a predetermined wavelength of output light in accordance with a control voltage signal applied to a scanning element. The scanning element located at the grating image of the spectrometer is a small mirror attached to the rotor of a galvanometer. The angular position of the galvanometer is accurately controlled by a closed-loop electronic control. The spectrum reflected from the mirror is passed through a slit to provide the output light of a predetermined wavelength. Selection of the waveform of the control signal allows the spectrometer to be operated as a dual wavelength spectrometer, to use a linear wavelength scan, or other wavelength scan patterns for absorbance analyses of a sample.
U.S. Pat. No. 4,264,205 Rapid Scan Spectral Analysis System Utilizing Higher Order Spectral Reflections Of Holographic Diffraction Gratings, Landa Apr. 28, 1981 And U.S. Pat. No. 4,285,596 Holographic Diffraction Grating System For Rapid Scan Spectral Analysis, Landa, Aug. 25, 1981 discloses an improved optical system for rapid, accurate spectral analysis of the reflectivity or transmissivity of samples. A concave holographic diffraction grating oscillating at high speed provides a rapid scanning of monochromatic light through a spectrum of wavelengths. The rapid scan by the grating enables the reduction of noise error by averaging over a large number of cycles. It also reduces the measurement time and thus prevents sample heating by excessive exposure to light energy. A filter wheel is rotated in the optical path and is synchronous with the grating.
U.S. Pat. No. 4,968,122 Galvanometer Gimbal Mount, Hlousek et. al., Nov. 6, 1990 discloses an improved mounting in which a rotating diffraction grating assembly directly connects the grating to the galvanometer that rotates the grating. The galvanometer is gimbal-mounted on a plate so that its position, and that of the grating, can be adjusted so that the plane of dispersion of the grating passes through a desired point when the grating is rotated.
U.S. Pat. No. 4,969,739 Spectrometer With Direct Drive High Speed Oscillating Grating, McGee, Nov. 13, 1990 discloses an optical grating oscillating at a high rate to scan a narrow wavelength band of light through the spectrum dispersed by a grating. The grating is connected integrally with the rotor of a motor, which is energized to oscillate its rotor between selected limits. High-speed oscillation is achieved by driving the motor with a pulse modulator having a duty cycle controlled by the motor speed. The direction that the motor is driven is controlled by the polarity of the pulse-modulated signal applied to a winding of the motor. The limits of the oscillation of the grating and the rate of rotation of the grating between the limits are selectively variable.
U.S. Pat. No. 5,488,240 Apparatus And Method For Rotating An Optical Element Using A Moving Coil In A Constant Magnetic Field, Hlousek et al., Jan. 30, 1996 discloses an apparatus and method for rotating an optical element, such as a diffraction grating or mirror, utilizing a moving coil actuator and an optical encoder to provide precise element position control. The moving coil actuator, which is coupled to the optical element, is comprised of a coil immersed in a magnetic field created by a pair of magnets. Current flowing in the coil windings causes the coil, and ultimately the optical element, to rotate. An optical encoder monitors the rotation of the element and provides rotation signals representative of the instantaneous element position to an actuator control circuit. The actuator control circuit phase shifts the rotation signals and compares the phase shifted rotation signals to a desired reference signal to generate position and velocity error signals.
The grating and, possibly, additional optical elements direct the light to the sample or target of interest. The angular displacement of the diffraction grating relative to the incoming light beam can be closely correlated with the individual wavelengths or range of wavelengths at which the sample is to be analyzed. By controlling the angular rotation and position of the diffraction grating, a range of wavelengths can be scanned at a known rate over a known time interval and, consequently, the individual wavelengths can then be distinguished as a function of time.
U.S. Pat. No. 5,981,956 Systems And Methods For Detection Of Labeled Materials, Stem, Nov. 9, 1999 discloses a reciprocating radiation direction system comprising a mirror selected from one of a galvanometric mirror, angularly oscillating mirror, or a rotating polyhedral mirror for scanning a focused excitation radiation across a surface of a substrate at a rate of at least 20 image lines/second. Labeled targets on a support synthesized with polymer sequences at known locations can be detected by exposing marked regions of sample to radiation from a source and detecting the emission there from, and repeating the steps of exposition and detection until the sample is completely examined.
U.S. Pat. No. 5,963,320 Active spectrometer, Brooks, et. al. Oct. 5, 1999 discloses a grating spectrometer employing digital control of an oscillating component (a mirror) and phase-locked digital recording of the intensity profile within the narrow spectral domain defined by an oscillation frequency. Flexible choice of oscillation frequency permits measurement in a quiet region of the noise spectrum. Reference waveforms acquired with the same instrument can be stored and later used to de-convolute a more complex spectrum. The use of multiple detector/slit combinations along a Rowland circle makes the spectrometer sensitive to specific atomic elements. A claim is made for an apparatus for providing one or more electrical signals representing a measurement of spectral similarity between an emission spectrum from a light source and a reference spectrum. It comprises an optical instrument that spectrally disperses an optical signal, a driver that induces relative movement in the dispersion direction between the optical signal""s imaged components in the image region and the template located in the image region, and a plurality of electro-optical sensors.
F. Vogt, U. Klocke, K. Rebstock, G. Schmidtke, V. Wander, M. Tack, Optical UV Derivative Spectrsocopy for Monitoring Gaseous Emissions, Applied Spectroscopy, November 1999, p. 1352. This paper""s FIG. 2 shows an optical grating which is oscillating rapidly as it more slowly rotates about its axis. That is, the grating rapidly oscillates about each wavelength while it more slowly sweeps over a range of optical wavelengths. While known devices address derivative spectrometry in a laboratory environment, there is no known derivative spectrometer that is able to operate under the conditions of a downhole environment. Thus, there is a need for a derivative spectrometer that is able to operate under the conditions of a downhole environment.
The present invention comprises a method of adapting derivative spectroscopy for use in a downhole environment and addresses problems that are inherent in this environment. Such problems include, but are not limited to, elevated temperatures and scattering from particles or other scatterers residing within dirty fluid samples. Elevated temperatures reduce the photodetector response for the same light level. Scatterers cause momentary jumps or spikes in the spectra, which, to first order, are simply temporary baseline offsets. Such repeated offsets make it difficult to obtain quantitative absorbance spectra of the pure (scatterer-free) fluid except by taking the first derivative with respect to wavelength, which removes baseline offsets. The present invention improves the photometric resolution by measuring the first derivative of the spectrum.
In the present invention, the spectrometer""s wavelength discrimination is provided by an optical filter whose color changes from one portion of the filter to another. By mechanically oscillating such a filter relative to one or more photodetectors, the amplitude of the signal produced by a photodetector will be proportional to the rate of change of the light transmission with wavelength (the first derivative spectra).
The derivative spectrometer of the present invention can operate by translation or vibration of a linear variable optical interference filter, back and forth along the plane of the filter. Alternatively, it can operate by rotational oscillation of a circular variable interference filter about some angle. The light can be filtered before entering the sample. Alternatively, we can let white light impinge on the sample, and filter the exiting light. In either case, the wavelength of light that eventually reaches the photodetector is oscillating. In a preferred embodiment, a vibrating actuator is provided to achieve the required oscillations about a given wavelength. Derivative spectrometry provides a higher resolution spectral measurement than normal methods of spectrometry. Through improved resolution, it is possible to accurately estimate the contamination percentage of a crude oil sample in real time as it is being pumped from the formation. (See co-pending applications Ser. No. 10/229,228 filed on Aug. 27, 2002 which is based on provisional application serial No. 60/359,895 both of which are incorporated herein by reference and U.S. patent application Ser. No. 10/119,492 filed on Apr. 10, 2002 by Rocco DiFoggio et al. entitled xe2x80x9cA Method and Apparatus for Downhole Refractometer And Attenuated Reflectance Spectrometerxe2x80x9d which is hereby incorporated herein by reference in its entirety.) Furthermore, the present invention enables determination of whether a contamination percentage is leveling off over time. High-resolution spectra provided by the present invention enables an improved estimation of the percentages of methane (natural gas), aromatic, olefins, saturates, and other crude oil properties. The present invention enables higher-resolution spectral measurements to determine a percentage contamination for samples and estimation of crude oil properties derived from the samples.
The present invention directly measures the derivative of the spectra, thereby minimizing baseline offset effect associated with scattering of light by contaminants found in the sample. Scattering, which can cause substantial baseline offsets, is particularly problematic for fluids withdrawn from unconsolidated formations. Such formations produce many fine particles that act as scatterers in a sample containing the particles. The present invention enables obtaining the first derivatives with respect to wavelength, which eliminates baseline offsets. These and other features and advantages of the present invention will be evident from reading the following description and figures for the present invention.